This invention relates to a vehicle air supply system and particularly to a system adaptable to occasionally supply large volumes of air at relatively low pressure for, for example, tyre inflation purposes.
A typical air supply system includes a compressor, a reservoir for pressurised air, and a drive operated demand valve to forward air from the reservoir for a desired purpose, for example to operate and maintain a braking system or pneumatic suspension system. The air supply system usually includes a dryer for compressor outlet air, and control means to take the compressor off load and regenerate the air dryer when the reservoir is at maximum pressure and the demand valve is closed. The compressor is typically taken off load by spragging a valve or by venting the compressor cylinder to atmosphere so that the compressor free wheels. Alternatively the compressor may include a clutch engageable with the compressor drive source, usually the vehicle engine. The air dryer is regenerated by passing a volume of dry air through the desiccant in reverse, and is typically triggered as the reservoir reaches maximum pressure, and regardless of the volume of air which has been dried.
In addition to the air being used e.g. for pneumatic suspension, the system may occasionally be required to forward relatively large volumes of air, i.e. volumes greater than the volume of the reservoir, at low pressure for, for example, tyre inflation. Typically the pressure of air required for tyre inflation will not exceed 2.5 bar. In such a situation the compressor is required to operate for longer than is normally required to fill the reservoir, and there exists the possibility that during this extended operation of the compressor the desiccant in the air dryer may become saturated especially if the air dryer is regenerated only at times when the reservoir reaches the maximum design pressure.
If however the air dryer is designed to cope with occasional demands for a large volume of air, the volume of desiccant will be very large, and the majority will be unused for most of the time. What is required is a solution which can accommodate occasional large volumes of air without enlarging the desiccant volume.
According to a first aspect the present invention there is provided an air supply system having a compressor, an air dryer, a reservoir adapted to receive air from the compressor via the air dryer, and control means operable to take the compressor off-load and regenerate the air dryer when a predetermined system condition is met, the control means being further operable to take the compressor off-load and regenerate the air dryer independently of said predetermined system condition being met.
In a preferred embodiment the additional control means are operable, to take the compressor off-load and to regenerate the air dryer in advance of said predetermined condition being met.
The control means is operable so as to prevent saturation of the air dryer desiccant in circumstances where the compressor is on load for an extended period of time, and thus likely to be pumping a large volume.
In a preferred embodiment the predetermined system condition is a reservoir pressure and a governor is adapted to operate when the reservoir reaches a target pressure. In such an embodiment the governor sends an off load/purge signal to the compressor and air dryer when the target pressure is reached. The signal may be a pressure signal. Preferably the pressure signal is communicated via a pressure line from the reservoir. The governor may, in a preferred embodiment, include a pressure sensitive valve in said pressure line.
The control means may be adapted to bypass the governor and send an off load/purge signal to the compressor and air dryer. In the case where signal is a pressure signal, the control means may include a bypass line for the governor, a valve positioned in said bypass line and a timer adapted to open said valve. The valve may be solenoid operated. The timer is thus adapted to open said valve and thereby send an off-load purge signal after a predetermined time period has elapsed. Preferably the timer is activated and suspended response to a predetermined system condition. The timer may be responsive to, for example, reservoir pressure or the operating condition of the compressor. The timer is preferably reset after each regeneration.
According to a second aspect of the present invention there is provided a vehicle air supply system having a compressor, an air dryer, a first reservoir adapted to receive air from the compressor via the air dryer, a second reservoir adapted to receive air from the first reservoir, and control means operable to cause a standard regeneration of the air dryer when a predetermined system condition is met, the control means being further operable to cause an intermediate regeneration of the air dryer in advance of said predetermined system condition being met.
In such an embodiment the control means includes a governor adapted to cause a standard regeneration and a governor bypass arrangement, adapted to cause an intermediate regeneration. The predetermined system condition is preferably a elevated reservoir pressure of the first reservoir.
The governor bypass arrangement preferably includes a delay means, for example a electronic timer, adapted to activate in response to a predetermined system parameter and cause an intermediate regeneration after a predetermined time period has elapsed. In one embodiment the predetermined system parameter is a reduced reservoir pressure of the second reservoir.
In such an embodiment a pressure sensitive switch is arranged so as to cause operation of the delay means in response to the pressure in the second reservoir dropping blow a predetermined value, for example when a large volume of air is forwarded from the second reservoir. In a preferred embodiment the pressure sensitive switch is adapted to operate a pneumatic signal piston of the delay means. The signal piston is arranged such that pressure from the first reservoir normally maintains the piston in a position whereby the delay means is deactivated. When the pressure switch is operated in response to a reduced pressure in the second reservoir, the pressure applied to the piston is removed, and the piston may be moved so as to activate the delay means. Movement of the piston may be achieved by the provision of a spring acting thereon.
In such an embodiment the pressure switch is arranged to operate a valve provided in a conduit extending between the first reservoir and the signal piston. Preferably the valve is a solenoid operated valve. When the pressure switch is operated the valve is movable from a first position wherein pressure from the first reservoir is communicated to the signal piston, and a second position wherein the conduit is blocked and the piston vented to atmosphere. It will be understood that the valve will revert to the first position when the pressure in the second reservoir rises above said predetermined value.
The delay means is configured cause an intermediate regeneration of the air dryer when a predetermined time period has elapsed. In a preferred embodiment the delay means is configured to send a pressure signal to the air dryer to take the compressor off-load and regenerate the air dryer. In such an embodiment the delay means are arranged so as to operate a valve in a conduit extending between the first reservoir and the air dryer. Preferably the valve is solenoid operated and movable between a first position where the conduit is closed, and a second position where the conduit is open.
In a preferred embodiment a protection valve is provided between the first and second reservoirs. The protection valve is adapted to maintain a predetermined minimum pressure in the first reservoir. The provision of such a protection valve ensures that the first reservoir holds sufficient pressure to operate the signal piston and cause intermediate regeneration of the air dryer.